<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    {{obj.foo}}
  </div>
</body>
<script src="../../dist/vue.js"></script>
<script>
  new Vue({
    el: '#app',
    data: {
      obj: {
        
      }
    },
    mounted() {
      this.$set(this.obj, 'foo', 'foooooo')
      // this.obj.foo = 'fooooo'
    }
  })
</script>
</html>